Macro ssz::impl_decode_via_from

macro_rules! impl_decode_via_from {
    ($impl_type: ty, $from_type: tt) => { ... };
}

Implements Decode for $impl_type using an implementation of From<$impl_type> for $from_type.

In effect, this allows for easy implementation of Decode for some type that implements a From conversion into another type that already has Decode implemented.